1.0 Field of the Invention
The present invention relates generally to gas turbine engines, and more particularly, to a system for cooling rotor disk posts, such as turbine rotor disk posts, of gas turbine engines.
2.0 Related Art
The highest temperatures in gas turbine engines are typically found in the combustor and the turbines. For instance, it is not uncommon for the temperature of the primary gas stream of the engine to exceed 2400.degree. F. at the entrance to the first stage blade of the high pressure turbine. The continuing demand for larger and more efficient gas turbine engines creates a requirement for increased turbine operating temperatures, with the metallurgical limitations of critical components such as rotor blades and disks in opposition to this requirement. For example, nickel-based alloys are commonly used in the manufacture of turbine rotor disks, with such alloys typically limited to maximum metal temperatures of approximately 1100.degree. F., which is considerably less than the maximum possible primary gas path temperature in the turbine. Consequently, there is a continuing need for novel approaches to provide thermal protection for components such as turbine rotor disks.
A turbine rotor disk is an annular component which rotates about the longitudinal axis of the engine and which supports a plurality of blades that extend radially into the primary gas stream. The disk typically includes a plurality of circumferentially alternating dovetail slots and posts disposed about the periphery of the disk, with each post formed by adjacent ones of the slots. Each disk dovetail slot is adapted to receive a corresponding dovetail portion, also referred to as a "fir tree" portion of a blade, with the blades being actually loaded into the disk. In addition to the dovetail portion, each blade includes a shank portion attached to and extending radially outward from the dovetail portion and a plate-like platform which radially separates the shank portion flora an airfoil portion of the blade extending radially into the primary gas stream flowpath. The outer surface of the blade platforms form a portion of the radially inner boundary of the primary gas stream flowpath, with the platform portions of adjacent stationary structures, such as nozzle segments, forming the remainder of the inner boundary. The Background Section of U.S. Pat. No. 5,388,962 issued to Wygle, et al., which is assigned to the assignee of the present invention and is expressly incorporated by reference herein in its entirety, provides a discussion of the need to cool the blades with conventional means such as compressor discharge air and further provides a discussion of the heat balance which determines the temperature of the disk posts. Wygle, et al. further explains that thermal isolation of the top of the disk posts from the hot air mixture existing in the cavity surrounding each disk post is an important part of the overall system for ensuring that the temperature of the disk posts do not exceed allowable limits.
As further explained in Wygle, et al., a known system to provide such disk post isolation has included shields located at the radially inward side of the blade platforms such that each shield spans the gap between platforms of adjacent blades to discourage ingestion of flowpath gases. This known system further includes cooling holes through the shank portions of the blade which communicate with the blade interior cooling passages in order to purge the cavities between the shanks of adjacent blades over each disk post. However, as noted in Wygle, et al., this system has the disadvantage of placing the holes in a highly stressed region of the blades, with the stress concentrations associated with the holes creating the potential for cracking and premature failure of the blades. This system has a further disadvantage due to the requirement of purging the relatively large cavities formed between shanks of adjacent blades and bounded at an outer end by the blade platforms and at an inner end by the top of one of the disk posts, which results in the use of a relatively high amount of compressor discharge cooling air and the associated engine performance penalty. Another disadvantage of this system is that the air injected into the cavities over the disk posts via the blade shank holes is significantly colder than the metal of the surrounding structures, particularly the blade platforms. As the colder "heavier" air is injected into the cavities it is subject to rotational effects. Centrifugal forces push the air radially outward so as to essentially bypass the disk posts. Accordingly, very little disk post cooling is accomplished. After the cavity cooling air is forced outward, radial recirculation occurs due to buoyancy forces caused by contact between the relatively hot blade platforms and the relatively cooler blade shank injected air. The disk posts are then washed, or scrubbed, by cavity cooling air which is at a much higher temperature than the cooling air flowing through the blade interior cooling passages, due to the contact of the cooling air with the blade platforms.
Also as noted in Wygle, et at., another known disk post isolation system has included the use of a structure commonly known as a seal body such as seal body assembly 28 illustrated in U.S. Pat. No. 5,201,849 issued to Chambers, et al., which is assigned to the assignee of the present invention and is expressly incorporated by reference herein in its entirety. Each seal body 28 of Chambers, et al. includes an aperture 32 in a forward end plate 36 opening into a diffusing hole 48 which is used to slowly drift forward cavity air over the top or radially outer surface of the corresponding disk post 24 so as to form an insulative layer of air over the disk post 24. However, as noted in Wygle, et al., this system is sensitive to manufacturing tolerances regarding the geometry of diffuser hole 48. If the geometry is not adequately controlled, the velocity of the forward cavity air passing over the outer surface of the disk post 24 may be unacceptably high, which may actually result in the temperature of the disk post 24 rising due to the associated convection heating from the forward cavity air.
The cooling system illustrated in FIGS. 3-5 of Wygle, et at., was developed to overcome the problems of the aforementioned known disk post thermal isolation systems. The Wygle, et al. system includes a seal body 31' positioned over the outer surface of each disk post 20. Each of the seal bodies includes a hole 176 formed through a forward portion of the seal body 31' for purposes of directing diverted blade cooling air onto an outer surface 33 of each disk post 20. The Wygle, et al. system further includes an annular blade retainer 48', having an increased radial height which may be seen by comparing retainer 48' in FIG. 3 of Wygle, et al. to retainer 48 illustrated in FIG. 1 of Wygle, et al. Blade retainer 48' is sealed at an inner end by seal 66 and at an outer end by seal 60 so as to prevent undesirable ingestion of gases from forward cavity 134 between the retainer 48' and the blade dovetail portions 24 and disk posts 20, and to prevent undesirable leakage of blade cooling air from plenums 94. It is noted that outer seal 60 is disposed radially outward of hole 176 formed through seal body 31'. During operation of engine 10, the blade cooling air is diverted from plenums 94 through slots 150 formed in each blade dovetail portion 24, so as to bypass inner seal 66, and is directed into plenum 160, formed in part by blade retainer 48'. The diverted cooling air then enters hole 176 and is directed across the top, or outer surface 33 of each disk post 20.
While the cooling system disclosed in FIGS. 3-5 of Wygle, et al. represents an improvement over previously existing known disk post thermal isolation systems, it is subject to the following disadvantages. The previously discussed increase in the radial height of the blade retainer 48' results in the radially outer end of the retainer 48' being closer to the hot gases of flowpath 32. Accordingly, it is more difficult to design a retainer 48' which will meet creep requirements due to the increased temperature of the outer end of the retainer 48' relative to prior retainers having a reduced radial height, such as retainer 48 illustrated in FIG. 1 of Wygle, et al.. Additionally, the increased height of retainer 48' results in an increased mass which in turn results in increased centrifugal forces acting on retainer 48' and an increase in bending stress in the fillet radius, indicated generally at E in FIG. 1 of the subject application, which is a partial reproduction of FIG. 3 of Wygle, et al., at the foot of the arm of retainer 48'. A further disadvantage of the Wygle, et al. cooling system is that the cooling air loses static pressure as it passes through each hole 176, prior to entering the corresponding thermal isolation chamber 144. This energy loss reduces the ability of the cooling air to adequately purge the thermal isolation chambers 144 so as to prevent ingestion of any hot air which may have scrubbed the underside of the blade platforms and exists in the cavities surrounding the seal bodies 31'.
In view of the foregoing, prior to the subject invention a need existed for a cooling system in a rotor assembly of a gas turbine engine to cool the top of rotor disk posts without compromising the structural integrity of the annular blade retainer used in the associated rotor assembly.